When concrete hardens or cures, calcium hydroxide (Ca(OH)2, or “free lime”) forms in small “deposits” throughout the substrate. Calcium hydroxide is a soft, weak material. Consequently, deposits of calcium hydroxide that are present at surfaces of concrete can, over time, compromise the integrity of the concrete.
Various compounds are known to be useful for reacting with calcium hydroxide to harden concrete, masonry, and stone. These compounds include silicas, silicates, siliconates, and mixtures of silicates and siliconates. The silicas, silicates, and/or siliconates of these materials react chemically with soft, weak calcium hydroxide to form stronger materials, densifying and strengthening the treated surface.
Silicas, silicates, and siliconates are widely available in water-based, or aqueous, solutions. These solutions are typically alkaline, or basic, with a pH of 10 or more. The relatively high pH “stabilizes” these solutions by imparting the suspended silica, silicate, and/or siliconate particles with a negative, repulsive charge that prevents the particles from aggregating and, thus, from falling out of, or precipitating from, the solution. When the pH of a conventional hardening solution drops below 10 (e.g., to 8), the effective negative charge on the particles decreases. As a result, the particles no longer repel one another. Instead, they begin to aggregate and precipitate. Stated another way, the solution destabilizes. From the foregoing, it is apparent that conventional concrete hardeners remain stable over a relatively narrow pH range.
When conventional hardeners that have been applied to concrete, masonry, or stone surfaces puddle, they leave a hard, white residue that is difficult to remove, often requiring additional chemical treatments, scrubbing, scraping, sanding, polishing, or other labor-intensive processes. As a consequence, an undesirably significant amount of labor is often required to treat concrete, masonry, or stone with conventional hardeners.